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How to Calculate Molecular Weight

Updated on December 21, 2013

Molecular weight (or molecular mass) is approximately equal to the relative molecular mass (Mr) of a molecule whose mass ratio is compared to 1/12 of carbon-12 Isotope. This is expressed as molar mass in chemical compounds by calculating the fractional contribution of the relative atomic mass of each of the elements in the compound.

Molecular weight in chemical reaction

The total amount of each of the reactants and products in a chemical reaction can be determined by calculating the molecular weight of the individual chemicals in relationship with their relative molecular mass and their amounts in the reaction. Consider a typical neutralization reaction,

H2SO4 + 2NaOH → Na2SO4 + 2H2O

While the molecular weight of Sodium Hydroxide (NaOH) and Water (H2O) are eventually multiplied by two (their number of moles in the reaction), Tetraoxosulphate (VI) Acid and Sodium Tetraoxosulphate (VI) salt are multiplied by one.

Molecular mass in H2SO4 will be 2(Hydrogen) + 1(Sulphur) + 4(Oxygen)

= 2(1) + 1(32) + 4(16)

= 2 + 32 + 64 = 98g/Mol.

While that of H2O is 2[2(Hydrogen) + 1(Oxygen)]

= 2[2(1) + 1(16)]

= 2(2 + 16)

= 2(18) = 36g/Mol.

Molecular weight of Ethanol

Ethanol exists in the chemical formula C2H5OH. The implication of this is that one mole of Ethanol contains two carbon atoms, six hydrogen atoms, and one oxygen atom. But the relative molecular mass of Carbon is 12, Hydrogen 1, and Oxygen 16. To calculate the Molecular weight of Ethanol, the relative molecular mass of each of the component elements must be substituted for the position of that element in the formula of the compound.

Therefore, relative molecular mass of Ethanol would be 2(12) + 5(1) + 1(16) + 1(1)

= 24 + 5 + 16 +1

= 46g/Mol

Calculation of molecular weight

The molecular weight or mass of a chemical compound is obtained from the sum of the proportion of masses of each of the constituent elements in the the chemical compound.

The molecular mass of a simple compound xy is equal to the sum of the individual mass of x and y. But for another compound t2g, the molecular weight is 2t+g. There are also instances where the molecular weight is multiplied by the number of moles of a compound taking place in a chemical reaction. This brings us to the question of molecular masses of compounds in a chemical reaction.


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    • James Agbogun profile image

      James Agbogun 5 years ago

      Tonipet, a primary priority is to simplify it as much as possible. I hope so many people from different parts of the world would find this article helpful. Thank you for stopping at my place.

    • Tonipet profile image

      Tonette Fornillos 5 years ago from The City of Generals

      I was always poor with computations, but somehow dropping by your molecular page has introduced me a basic arithmetic of a chemical reaction. Informative page.